1. Field of the Invention
The present invention relates to an active centering apparatus, particularly suitable for use in cylindrical grinding machines and the like, for sensing force levels and vibrational frequencies and for providing force and displacement compensation. More particularly, the present invention relates to an active centering apparatus having an imbedded shear load sensor and an imbedded actuator.
2. Prior Art
Machining operations, such as cylindrical grinding, generate undesirable force levels and vibrational frequencies between the workpiece and the machine tool. These force levels and vibrational frequencies have a detrimental effect on surface finishes, precision tolerances, and cycle times. For example, a harmonic disturbance force causes a periodic motion between a grinding wheel and the workpiece. The periodic motion causes the workpiece to develop a wavy shape. The periodic motion may be caused by grinding wheel imbalance, bearing vibration, motor vibration, etc.
Older machines, still in use today, operate slowly and are difficult to adjust. For example, retracting the grinding wheel after finishing is slow due to the mass of the grinding wheel and spindle assembly and the use of conventional hydraulic pistons. The slow retraction decreases the quality of the surface finish and increases cycle times. As another example, a misalignment often exists between the workpiece and the grinding wheel, resulting in a taper in the finished workpiece. Correcting for machine misalignment is a time consuming labor intensive technique.
In addition, set-up and calibration time between similar and different workpieces is also time consuming and labor intensive. Furthermore, many machines do not include any type of operator feed-back.
Some devices have been proposed that have a sensor to measure an applied tool force or workpiece displacement and an actuator to control relative movement between the workpiece and the machine or tool. For example, U.S. Pat. No. 5,054,244, issued Oct. 8, 1991 to Takamatsu et al. discloses a polishing apparatus with a rotating polishing tool applied to a workpiece held by a movable table. A load cell or strain gauge is disposed under the table for detecting an applied tool force. The load cell generates a signal which is sent to a controller. The controller generates a signal which is sent to a piezoelectric member, also disposed under the table, which moves the table in response to the workpiece.
Similarly, U.S. Pat. No. 4,590,573, issued May 20, 1986 to Hahn discloses a grinding machine with a workpiece rotatably mounted to a workhead, which in turn is mounted on a base, and an abrasive tool rotatably mounted to a wheelhead. The wheelhead is mounted to a table, which in turn is movably mounted on the base. A feed means moves the tool and wheelhead on the base with respect to the workpiece and workhead. A sensor is disposed on the wheelhead for sensing grinding force. The sensor generates a signal which is sent to a controller. The controller, in turn, generates a signal which is sent to the feed means to move the table, and thus the wheelhead and tool.
Similarly, U.S. Pat. No. 4,602,459, issued Jul. 29, 1986 to Drits et al. discloses a hinged table holding a workpiece against a grinding wheel. A sensor is disposed under the table for sensing the position of the table. The sensor generates a signal which is sent to a controller. The controller generates a signal which is sent to a piezoelectric actuator to pivot the table, and thus the workpiece, with respect to the grinding wheel.
Some of the above devices, however, require special tables to transmit the force or displacement applied by the tool to a sensor and to transmit the force and displacement applied by the actuation to the workpiece. All the above devices respond by activating an actuator to move a table and cause relative movement between the workpiece and a tool. None of the above devices address cylindrical grinding machines or similar machines that support the workpiece between spindles or centers.
In addition, many older machines are still in use today and do not include any type of force sensors or controls. These older machines comprise many different models from different manufactures. Thus, numerous different retro-fit designs would be required to provide for customized tables to fit the different physical configurations. In addition, retro-fitting older machines would require a great deal of machine down-time. Therefore, custom modifications for existing machines seems impractical.
Therefore, it would be advantageous to develop a device capable of sensing and/or responding to undesirable force levels and vibrational frequencies. It also would be advantageous to develop such a device capable of being used with existing machines without extensive retro-fitting. It also would be advantageous to develop such a device for providing real-time force measurement without disturbing the work space in order to determine undesirable force levels and frequencies. It also would be advantageous to develop such a device capable of adjusting for relative taper between the workpiece and the tool. In addition, it would be advantageous to develop such a device capable of rapidly removing the workpiece from the tool.